Only a Monotheistic Soul Enters Paradise
The statement in the chapter title is a theological foundation in Islam. It means that only a soul that is fully submitted to its Lord can enter Paradise. As has already been made clear multiple times, this is precisely the meaning of the word “Muslim.”
Only the monotheist, who worships nothing besides the one Lord, can attain his place in Paradise. In this context, Muslim and Islam refer to the previously mentioned “universal Islam.” This is the message of all prophets عليهم السلام. For this reason, all previous prophets and their followers are referred to as Muslims in the Qur'an, as can be seen in many verses of the Qur'an.
What is stated in the text thus applies to all those who followed their respective prophet عليهم السلام and accepted his message, regardless of which prophet it was. For this reason, every Muslim must be convinced that among the followers of Moses and Jesus, meaning the Jews and Christians, there were undoubtedly true monotheists who will also enter Paradise – and they were not few in number. This is a theological certainty and rejecting it would be kufr.
Al-Bukhari رحمه الله narrates in his Sahih collection from Abu Hurayrah رضي الله عنه, and in another hadith from 'Abdullah ibnu Mas'ud, an interesting statement of the Prophet صلى الله عليه وسلم regarding this matter:
لَا يَدْخُلُ الْجَنَّةَ إِلَّا نَفْسٌ مُسْلِمَةٌ
Only and exclusively a Muslim soul enters Paradise.
The word “Muslimah” is used here as a characteristic in the sense of “monotheistic” and “fully and completely submitted to Allah.” Clearly, this description does not apply to the polytheist.